Stephen Krashen has published over 500 articles and a dozen scholarly books in the fields of literacy, language acquisition, neurolinguistics, and bilingual education. Many of these publications are available for free download at sdkrashen.com. He no longer writes books because nobody can afford them. Nevertheless, he is the most frequently cited scholar in the field of language education.
He is best known for the Comprehension Hypothesis, the idea that we do not acquire language by speaking or writing, or by studying grammar rules. We acquire language when we understand what we hear and what we read, when we get “comprehensible input.” The ability to acquire language in this way does not disappear at puberty or bar mitzvah: It remains strong our entire lives.
He has arrived at his conclusions from research, both his own and others, and through case histories, including his own. When he was in high school, his French teacher gave him a passing grade in French under the condition that he never take a French class again at that school. Today, he speaks French quite well, as well as several other languages.